Answer to Question 1
A motor unit consists of all of the skeletal muscle fibers controlled by a single motor neuron. They may have as few as 10 fibers (eye muscles) or as many as 10,000 (back muscles).
Answer to Question 2
This is a case of competition between acetylcholine and the antibody. To make the patient's acetylcholine more effective, a drug that inhibits acetylcholinesteras e would slow the breakdown of acetylcholine, relieving some of the weakness. An acetylcholine blocker would be worse than doing nothing.